Detecting sleep

ABSTRACT

A system includes one or more sensors and a processor. Each of the sensors generates a signal as a function of at least one physiological parameter of a patient that may discernibly change when the patient is asleep. The processor monitors the physiological parameters, and determines whether the patient is asleep based on the parameters. In some embodiments, the processor determines plurality of sleep metric values, each of which indicates a probability of the patient being asleep, based on each of a plurality of physiological parameters. The processor may average or otherwise combine the plurality of sleep metric values to provide an overall sleep metric value that is compared to a threshold value in order to determine whether the patient is asleep.

This application claims the benefit of U.S. Provisional Application No.60/553,771, filed Mar. 16, 2004, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The invention relates to medical devices, and to techniques fordetermining whether a patient is asleep.

BACKGROUND

The ability to determine whether a patient is asleep is useful in avariety of medical contexts. In some situations, the ability todetermine whether a patient is asleep is used to diagnose conditions ofthe patient. For example, the amount of time that patients sleep, theextent of arousals during sleep, and the times of day that patientssleep have been used to diagnose sleep apnea. Such sleep informationcould also be used to diagnose psychological disorders, such asdepression and mania.

In other situations, a determination as to whether a patient is asleepis used to control delivery of therapy to the patient. For example,neurostimulation or drug therapies can be suspended when the patient isasleep, or the intensity/dosage of the therapies can be reduced when apatient is asleep. As another example, the rate response settings of acardiac pacemaker may be adjusted to less aggressive settings when thepatient is asleep so that the patient's heart will not be paced at aninappropriately high rate during sleep. In these examples, therapy maybe suspended or adjusted when the patient is asleep to avoid patientdiscomfort, or to conserve a battery and/or contents of a fluidreservoir of an implantable medical device when the therapy may beunneeded or ineffective. However, in other cases, a therapy intended tobe delivered when the patient is asleep, such as therapy intended toprevent or treat sleep apnea, is delivered based on a determination thatthe patient is asleep.

Existing techniques for determining whether a patient is asleep includemonitoring the electroencephalogram (EEG) of the patient to identifybrain wave activity indicative of sleep. However, EEG monitoringtypically requires that an array of electrodes be placed on a patient'sscalp and coupled to an external monitoring device, and is most oftenperformed in a clinic setting. Generally, an implantable medical devicemay only be used to monitor a patient's EEG in the rare cases when it iscoupled to electrodes implanted within the brain of the patient.Consequently, EEG monitoring is generally unsuitable as a technique fordetermining whether a patient is asleep in order to control therapy, orfor long-term monitoring of the patient's sleep/wake cycle.

Existing techniques employed by implantable medical devices to determinewhether a patient is asleep include monitoring the patient's respirationrate, respiration rate variability, and activity level. Each of thesephysiological parameters may be an inaccurate indicator of whether apatient is asleep. For example, from the perspective of thesephysiological parameters, it may appear that a patient is sleeping when,instead, the patient is merely lying down in a relaxed state. As anotherexample, respiration rate and respiration rate variability, for example,may fail to accurately indicate that the patient is asleep when thepatient suffers from a breathing disorder, such as Cheyne-Stokessyndrome.

SUMMARY

In general, the invention is directed to techniques for determiningwhether a patient is asleep. In some embodiments, the invention isdirected to techniques that involve determination of values of one ormore sleep metrics that indicate a probability of a patient being asleepbased on the current value of one or more physiological parameters ofthe patient. Use of a plurality of sleep metrics, in particular, mayallow for a more accurate determination of whether a patient is asleep.

A system according to the invention includes one or more sensors and aprocessor. Each of the sensors generates a signal as a function of atleast one physiological parameter of a patient that may discerniblychange when the patient is asleep. Exemplary physiological parametersinclude activity level, posture, heart rate, respiration rate,respiratory volume, blood pressure, blood oxygen saturation, partialpressure of oxygen within blood, partial pressure of oxygen withincerebrospinal fluid, muscular activity, core temperature, arterial bloodflow, and galvanic skin response.

The processor monitors the physiological parameters based on the signalsgenerated by the sensors, and determines whether the patient is asleepbased on values for the physiological parameters. The value for aphysiological parameter may be a current, mean or median value for theparameter. In some embodiments, the processor may additionally oralternatively determine whether the patient is asleep based on thevariability of one or more of the physiological parameters.

In some embodiments, the processor determines a value of a sleep metricthat indicates a probability of the patient being asleep based on aphysiological parameter. In particular, the processor may apply afunction or look-up table to the current value and/or variability of thephysiological parameter to determine the sleep metric value. Theprocessor may compare the sleep metric value to a threshold value todetermine whether the patient is asleep. In some embodiments, theprocessor may compare the sleep metric value to each of a plurality ofthresholds to determine the current sleep state of the patient, e.g.,rapid eye movement (REM), or one of the nonrapid eye movement (NREM)states (S1, S2, S3, S4). Because they provide the most “refreshing” typeof sleep, the ability to determine whether the patient is in one of theS3 and S4 sleep states may be, in some embodiments, particularly useful.

Further, in some embodiments the processor may determine a sleep metricvalue for each of a plurality of physiological parameters. In otherwords, the processor may apply a function or look-up table for eachparameter to the current value for that parameter in order to determinethe sleep metric value for that parameter. The processor may average orotherwise combine the plurality of sleep metric values to provide anoverall sleep metric value for comparison to the threshold values. Insome embodiments, a weighting factor may be applied to one or more ofthe sleep metric values. One or more of functions, look-up tables,thresholds and weighting factors may be selected or adjusted by a userin order to select or adjust the sensitivity and specificity of thesystem in determining whether the patient is asleep.

In some embodiments, the processor is included as part of a medicaldevice, such as an implantable medical device. The sensors may also beincluded within the medical device, coupled to the medical device by oneor more leads, or in wireless communication with the medical device. Themedical device may control delivery of therapy to the patient based onthe determination as to whether the patient is asleep, or may storeinformation indicating when the patient is asleep for later retrievaland analysis by user. In some embodiments, the medical device mayinstead use the one or more sleep metric values to control delivery oftherapy, or may store one or more sleep metric values. In someembodiments, information relating to the patient's sleep patterns may beused to diagnose sleep or psychological disorder, or may be used toevaluate the effectiveness of a therapy delivered to the patient.

In one embodiment, the invention is directed to a method in which aplurality of physiological parameters of a patient are monitored and avalue of a sleep metric that indicates a probability of the patientbeing asleep is determined based on the physiological parameters.

In another embodiment, the invention is directed to a medical systemthat comprises at least one sensor and a processor. The sensor generatesa signal as a function of at least one physiological parameter of apatient. The processor receives the signal from the sensor, monitors aplurality of physiological parameters of the patient, and determines avalue of a sleep metric that indicates a probability of the patientbeing asleep based on the physiological parameters.

In another embodiment, the invention is directed to a medical systemthat comprises means for monitoring a plurality of physiologicalparameters of a patient and means for determining a value of a sleepmetric that indicates a probability of the patient being asleep based onthe physiological parameters.

In another embodiment, the invention is directed to a method in which aphysiological parameter of a patient is monitored via an implantablemedical device, and whether the patient is asleep is determined based onthe physiological parameter. The physiological parameter comprises oneof heart rate, blood pressure, blood oxygen saturation, muscularactivity and core temperature.

In another embodiment, the invention is directed to a medical systemthat comprises a sensor and an implantable medical device. The sensorgenerates a signal as a function of a physiological parameter of apatient. The physiological parameter comprises one of heart rate, bloodpressure, blood oxygen saturation, muscular activity, core temperature,arterial blood flow, and galvanic skin response. The implantable medicaldevice includes a processor that monitors the physiological parameterbased on the signal and determines whether the patient is asleep basedon the physiological parameter.

In another embodiment, the invention is directed to a computer-readablemedium comprising program instructions. The instructions cause aprogrammable processor to monitor a plurality of physiologicalparameters of a patient, and determine a value of a sleep metric thatindicates a probability of the patient being asleep based on thephysiological parameters.

The invention may be capable of providing one or more advantages. Forexample, the invention provides techniques for determining a sleep stateof a patient that may be implemented in an implantable medical device.Further, the techniques provided by the invention may include analysisof a variety of physiological parameters not previously used indetermining whether a patient is asleep. Where it is desired to detectsleep via an implantable medical device, the ability to determinewhether a patient is sleeping based on these physiological parametersmay increase the number of implantable medical device types in which theinvention may be implemented, i.e., the invention may be implemented ina variety of types of implantable medical devices which include or maybe easily modified to include sensors capable of generating a signalbased on such physiological parameters.

Monitoring a plurality of physiological parameters according to someembodiments, rather than a single parameter, may allow for a moreaccurate determination of whether a patient is asleep than is availablevia existing implantable medical devices. Use of sleep metrics thatindicate a probability of the patient being asleep for each of aplurality of physiological parameters may further increase thereliability with which an implantable medical device may determinewhether a patient is asleep. In particular, rather than a binary sleepor awake determination for each of a plurality of parameters, sleepmetric values for each of a plurality of parameters may be combined toyield an overall sleep metric value that may be compared to a thresholdto determine whether the patient is asleep. In other words, failure ofany one physiological parameter to accurately indicate whether a patientis sleeping may be less likely to prevent the implantable medical devicefrom accurately indicating whether the patient is sleeping whenconsidered in combination with other physiological parameters.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example system includingan implantable medical device that determines whether a patient isasleep according to the invention.

FIG. 2 is a block diagram further illustrating the system of FIG. 1.

FIG. 3 is a block diagram illustrating a memory within an implantablemedical device of the system of FIG. 1.

FIG. 4 is a flowchart illustrating an example technique for determiningwhether a patient is asleep.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram illustrating an example system 10 thatincludes an implantable medical device (IMD) 14 that determines whethera patient 12 is asleep according to the invention. In the illustratedexample system, IMD 14 takes the form of an implantable neurostimulatorthat delivers neurostimulation therapy in the form of electrical pulsesto patient 12. However, the invention is not limited to implementationvia an implantable neurostimulator, or even to implementation via anIMD.

For example, in some embodiments of the invention, an implantable pumpor implantable cardiac pacemaker may determine whether a patient isasleep. In other embodiments, a programming device may determine whenpatient 12 is asleep based on information collected by a medical device.In other words, any implantable or external device may determine whethera patient is asleep according to the invention.

In the illustrated example, IMD 14 delivers neurostimulation therapy topatient 12 via leads 16A and 16B (collectively “leads 16”). Leads 16may, as shown in FIG. 1, be implanted proximate to the spinal cord 18 ofpatient 12, and IMD 14 may deliver spinal cord stimulation (SCS) therapyto patient 12 in order to, for example, reduce pain experienced bypatient 12. However, the invention is not limited to the configurationof leads 16 shown in FIG. 1 or the delivery of SCS therapy. For example,one or more leads 16 may extend from MD 14 to the brain (not shown) ofpatient 12, and IMD 14 may deliver deep brain stimulation (DBS) therapyto patient 12 to, for example, treat tremor or epilepsy. As furtherexamples, one or more leads 16 may be implanted proximate to the pelvicnerves (not shown) or stomach (not shown), and IMD 14 may deliverneurostimulation therapy to treat incontinence or gastroparesis.

IMD 14 delivers therapy according to a set of therapy parameters thatdefine the delivered therapy. In embodiments where IMD 14 deliversneurostimulation therapy in the form of electrical pulses, theparameters for each of the parameter sets may include voltage or currentpulse amplitudes, pulse widths, pulse rates, and the like. Further, eachof leads 16 includes electrodes (not shown in FIG. 1), and theparameters may include information identifying which electrodes havebeen selected for delivery of pulses, and the polarities of the selectedelectrodes.

System 10 also includes a clinician programmer 20. A clinician (notshown) may use clinician programmer 20 to program neurostimulationtherapy for patient 12. Clinician programmer 20 may, as shown in FIG. 1,be a handheld computing device. Clinician programmer 20 includes adisplay 22, such as a LCD or LED display, to display information to auser. Clinician programmer 20 may also include a keypad 24, which may beused by a user to interact with clinician programmer 20. In someembodiments, display 22 may be a touch screen display, and a user mayinteract with clinician programmer 20 via display 22. A user may alsointeract with clinician programmer 20 using peripheral pointing devices,such as a stylus, mouse, or the like. Keypad 24 may take the form of analphanumeric keypad or a reduced set of keys associated with particularfunctions.

System 10 also includes a patient programmer 26, which also may, asshown in FIG. 1, be a handheld computing device. Patient 12 may usepatient programmer 26 to control the delivery of neurostimulationtherapy by IMD 14. Patient programmer 26 may also include a display 28and a keypad 30, to allow patient 12 to interact with patient programmer26. In some embodiments, display 26 may be a touch screen display, andpatient 12 may interact with patient programmer 26 via display 28.Patient 12 may also interact with patient programmer 26 using peripheralpointing devices, such as a stylus or mouse.

IMD 14, clinician programmer 20 and patient programmer 26 may, as shownin FIG. 1, communicate via wireless communication. Clinician programmer20 and patient programmer 26 may, for example, communicate via wirelesscommunication with IMD 14 using RF telemetry techniques known in theart. Clinician programmer 20 and patient programmer 26 may communicatewith each other using any of a variety of local wireless communicationtechniques, such as RF communication according to the 802.11 orBluetooth specification sets, infrared communication according to theIRDA specification set, or other standard or proprietary telemetryprotocols.

Clinician programmer 20 and patient programmer 26 need not communicatewirelessly, however. For example, programmers 20 and 26 may communicatevia a wired connection, such as via a serial communication cable, or viaexchange of removable media, such as magnetic or optical disks, ormemory cards or sticks. Further, clinician programmer 20 may communicatewith one or both of IMD 14 and patient programmer 26 via remotetelemetry techniques known in the art, communicating via a local areanetwork (LAN), wide area network (WAN), public switched telephonenetwork (PSTN), or cellular telephone network, for example.

As mentioned above, IMD 14 is capable of determining whether patient 12is asleep. Specifically, as will be described in greater detail below,IMD 14 monitors a plurality of physiological parameters of patient 12that may discernibly change when patient 12 is asleep, and determineswhether patient 12 is asleep based on values of the physiologicalparameters. The value for a physiological parameter may be a current,mean or median value for the parameter. In some embodiments, IMD 14 mayadditionally or alternatively determine whether patient 12 is asleepbased on the variability of one or more of the physiological parameters.IMD 14 includes, is coupled to, or is in wireless communication with oneor more sensors, and monitors the physiological parameters via thesensors.

Exemplary physiological parameters that may be monitored by IMD 14include activity level, posture, heart rate, respiration rate,respiratory volume, blood pressure, blood oxygen saturation, partialpressure of oxygen within blood, partial pressure of oxygen withincerebrospinal fluid, muscular activity, core temperature, and arterialblood flow. In some external medical device embodiments of theinvention, galvanic skin response may additionally or alternatively bemonitored. Some of the parameters, such as activity level, heart rate,respiration rate, respiratory volume, blood pressure, arterial oxygensaturation and partial pressure, partial pressure of oxygen in thecerebrospinal fluid, muscular activity, core temperature, arterial bloodflow, and galvanic skin response may be at low values when patient 12 isasleep. Further, the variability of at least some of these parameters,such as heart rate and respiration rate, may be at a low value when thepatient is asleep. Information regarding the posture of patient 12 willmost likely indicate that patient 12 is lying down when patient 12 isasleep.

In some embodiments, IMD 14 determines a value of one or more sleepmetrics based on a value of one or more physiological parameters ofpatient 12. A sleep metric value may be a numeric value that indicatesthe probability that patient 12 is asleep. In some embodiments, thesleep metric value may be a probability value, e.g., a number within therange from 0 to 1.

In particular, the IMD 14 may apply a function or look-up table to thecurrent, mean or median value, and/or the variability of thephysiological parameter to determine a value of the sleep metric. IMD 14may compare the sleep metric value to a threshold value to determinewhether the patient is asleep. In some embodiments, the IMD 14 maycompare the sleep metric value to each of a plurality of thresholds todetermine the current sleep state of the patient, e.g., rapid eyemovement (REM), S1, S2, S3, or S4. Because they provide the most“refreshing” type of sleep, the ability to determine whether the patientis in one of the S3 and S4 sleep states may be, in some embodiments,particularly useful.

Further, in some embodiments IMD 14 may determine a sleep metric valuefor each of a plurality of physiological parameters. In other words, IMD14 may apply a function or look-up table for each parameter to a valuefor that parameter in order to determine the sleep metric value for thatparameter. IMD 14 may average or otherwise combine the plurality ofsleep metric values to provide an overall sleep metric value forcomparison to the threshold values. In some embodiments, IMD 14 mayapply a weighting factor to one or more of the sleep metric values priorto combination. One or more of functions, look-up tables, thresholds andweighting factors may be selected or adjusted by a user, such as aclinician via programmer 20 or patient 12 via programmer 26, in order toselect or adjust the sensitivity and specificity of IMD 14 indetermining whether patient 12 is asleep.

Monitoring a plurality of physiological parameters according to someembodiments, rather than a single parameter, may allow IMD 14 todetermine whether patient 12 is asleep with more accuracy than existingimplantable medical devices. Use of sleep metric values that indicate aprobability of the patient being asleep for each of a plurality ofphysiological parameters may further increase the accuracy with whichIMD 14 may determine whether patient 12 is asleep. In particular, ratherthan a binary sleep or awake determination for each of a plurality ofparameters, sleep metric values for each of a plurality of parametersmay be combined to yield an overall sleep metric value that may becompared to a threshold to determine whether patient 12 is asleep. Inother words, failure of any one physiological parameter to accuratelyindicate whether a patient is sleeping may be less likely to prevent IMD14 from accurately indicating whether patient 12 is sleeping whenconsidered in combination with other physiological parameters.

IMD 14 may control delivery of therapy to patient 12 based on thedetermination as to whether patient 12 is asleep. For example, IMD 14may suspend delivery of neurostimulation or reduce the intensity ofdelivered neurostimulation when patient 14 is determined to be asleep.In other embodiments, IMD 14 may suspend or reduce intensity of drugdelivery, or may reduce the aggressiveness of rate response for cardiacpacing when patient 12 is determined to be asleep. In still otherembodiments, IMD 14 may initiate delivery of a therapy, such as atherapy to treat or prevent sleep apnea, when patient 12 is determinedto be asleep.

In some embodiments, IMD 14 stores information indicating when patient12 is asleep, which may be retrieved for analysis by a clinician viaprogrammer 20, for example. The clinician may use the sleep informationto diagnose conditions of patient 12, such as sleep apnea orpsychological disorders. Information relating to the sleep patterns ofpatient 12 may in other situations indicate the effectiveness of adelivered therapy and/or the need for increased therapy. Some ailmentsof patient 12, such as chronic pain, tremor, gastrointestinal disorders,incontinence, congestive heart failure, and sleep apnea may disturb orhinder the sleep or patient 12, or, in some cases, inadequate ordisturbed sleep may increase the symptoms of these ailments.

IMD 14 may collect information relating to the sleep patterns of patient12, which may be retrieved by a clinician via programmer 20 and used toevaluate the effectiveness of a therapy delivered to patient 12 for suchan ailment, or to indicate the need for an additional therapy to improvethe sleep pattern of patient 12. In some cases, IMD 14 may evaluate suchcollected sleep information and automatically adjust a therapy for sucha condition based on the evaluation. Further information regardingevaluation of a therapy based on sleep information collected by an IMDmay be found in a commonly-assigned and copending U.S. patentapplication by Ken Heruth and Keith Miesel, entitled “COLLECTING SLEEPQUALITY INFORMATION VIA A MEDICAL DEVICE,” which is filed on Apr. 15,2004 and assigned attorney docket number 1023-250US01. Furtherinformation regarding automatic control of a therapy based on sleepinformation collected by an IMD may be found in a commonly-assigned andcopending U.S. patent application by Ken Heruth and Keith Miesel,entitled “CONTROLLING THERAPY BASED ON SLEEP QUALITY,” which is filed onApr. 15, 2004 and assigned attorney docket number 1023-363US01. Theentire content of both of these applications is incorporated herein byreference.

FIG. 2 is a block diagram further illustrating system 10. In particular,FIG. 2 illustrates an example configuration of IMD 14 and leads 16A and16B. FIG. 2 also illustrates sensors 40A and 40B (collectively “sensors40”) that generate signals as a function of one or more physiologicalparameters of patient 12. IMD 14 monitors the signals to determinewhether patient 12 is asleep.

IMD 14 may deliver neurostimulation therapy via electrodes 42A-D of lead16A and electrodes 42E-H of lead 16B (collectively “electrodes 42”).Electrodes 42 may be ring electrodes. The configuration, type and numberof electrodes 42 illustrated in FIG. 2 are merely exemplary. Forexample, leads 16A and 16B may each include eight electrodes 42, and theelectrodes 42 need not be arranged linearly on each of leads 16A and16B.

Electrodes 42 are electrically coupled to a therapy delivery module 44via leads 16A and 16B. Therapy delivery module 44 may, for example,include a pulse generator coupled to a power source such as a battery.Therapy delivery module 44 may deliver electrical pulses to patient 12via at least some of electrodes 42 under the control of a processor 46,which controls therapy delivery module 44 to deliver neurostimulationtherapy according to a set of therapy parameters, which may be one of aplurality of therapy parameter sets stored in memory 48. However, theinvention is not limited to implantable neurostimulator embodiments oreven to IMDs that deliver electrical stimulation. For example, in someembodiments a therapy delivery module 44 of an IMD may include a pump,circuitry to control the pump, and a reservoir to store a therapeuticagent for delivery via the pump.

Processor 46 may include a microprocessor, a controller, a digitalsignal processor (DSP), an application specific integrated circuit(ASIC), a field-programmable gate array (FPGA), discrete logiccircuitry, or the like. Memory 48 may include any volatile,non-volatile, magnetic, optical, or electrical media, such as a randomaccess memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM),electrically-erasable programmable ROM (EEPROM), flash memory, and thelike. In some embodiments, memory 48 stores program instructions that,when executed by processor 46, cause IMD 14 and processor 46 to performthe functions attributed to them herein.

Each of sensors 40 generates a signal as a function of one or morephysiological parameters of patient 12. Although shown as including twosensors 40, system 10 may include any number of sensors. As illustratedin FIG. 2, sensors 40 may be included as part of IMD 14, or coupled toIMD 14 via leads 16. Sensors 40 may be coupled to IMD 14 via therapyleads 16A and 16B, or via other leads 16, such as lead 16C depicted inFIG. 2. In some embodiments, a sensor located outside of IMD 14 may bein wireless communication with processor 46.

As discussed above, exemplary physiological parameters of patient 12that may be monitored by IMD 14 to determine values of one or more sleepmetrics include activity level, posture, heart rate, respiration rate,respiratory volume, blood pressure, blood oxygen saturation, partialpressure of oxygen within blood, partial pressure of oxygen withincerebrospinal fluid, muscular activity, core temperature, and arterialblood flow. Further, as discussed above, in some external medical deviceembodiments of the invention galvanic skin response may additionally oralternatively be monitored. The detected values of these physiologicalparameters of patient 12 may discernibly change when patient 12 fallsasleep or wakes up. In particular, these physiological parameters may beat low values when patient 12 is asleep. Further, the variability of atleast some of these parameters, such as heart rate and respiration rate,may be at a low value when the patient is asleep. Sensors 40 may be ofany type known in the art capable of generating a signal as a functionof one or more of these parameters.

For example, sensors 40 may include electrodes located on leads orintegrated as part of the housing of IMD 14 that generate an electrogramsignal as a function of electrical activity of the heart of patient 12,and processor 46 may monitor the heart rate of patient 12 based on theelectrogram signal. In other embodiments, a sensor may include anacoustic sensor within IMD 14, a pressure or flow sensor within thebloodstream or cerebrospinal fluid of patient 12, or a temperaturesensor located within the bloodstream of patient 12. The signalsgenerated by such sensors may vary as a function of contraction of theheart of patient 12, and can be used by IMD 14 to monitor the heart rateof patient 12.

Sensors 40 may include one or more accelerometers, gyros, mercuryswitches, or bonded piezoelectric crystals that generate a signal as afunction of patient activity, e.g., body motion, footfalls or otherimpact events, and the like. Sensors 40 may also include a plurality ofaccelerometers, gyros, or magnetometers oriented orthogonally thatgenerate signals that indicate the posture of patient 12. In addition tobeing oriented orthogonally with respect to each other, each of sensors40 used to detect the posture of patient 12 may be generally alignedwith an axis of the body of patient 12. When accelerometers, forexample, are aligned in this manner, the magnitude and polarity of DCcomponents of the signals generate by the accelerometers indicate theorientation of the patient relative to the Earth's gravity, e.g., theposture of patient 12. Further information regarding use of orthogonallyaligned accelerometers to determine patient posture may be found in acommonly-assigned U.S. Pat. No. 5,593,431, which issued to Todd J.Sheldon.

In some embodiments, sensors 40 may include an electrode pair, includingone electrode integrated with the housing of IMD 14 and one ofelectrodes 42, that generates a signal as a function of the thoracicimpedance of patient 12, which varies as a function of respiration bypatient 12. In other embodiments, sensors 40 may include a strain gauge,bonded piezoelectric element, or pressure sensor within the blood orcerebrospinal fluid that generates a signal that varies based on patientrespiration. An electrogram generated by electrodes as discussed abovemay also be modulated by patient respiration, and may be used as anindirect representation of respiration rate.

Sensors 40 may include electrodes that generate an electromyogram (EMG)signal as a function of muscle electrical activity, or may include anyof a variety of known temperature sensors to generate a signal as afunction of a core temperature of patient 12. Such electrodes andtemperature sensors may be incorporated within the housing of IMD 14, orcoupled to IMD 14 via leads. Sensors 40 may also include a pressuresensor within, or in contact with, a blood vessel. The pressure sensormay generate a signal as a function of the a blood pressure of patient12, and may, for example, comprise a Chronicle Hemodynamic Monitor™commercially available from Medtronic, Inc. of Minneapolis, Minn.

Sensors 40 may also include optical pulse oximetry sensors or Clarkdissolved oxygen sensors located within, as part of a housing of, oroutside of IMD 14, which generate signals as a function of blood oxygensaturation and blood oxygen partial pressure respectively. In someembodiments, system 10 may include a catheter with a distal portionlocated within the cerebrospinal fluid of patient 12, and the distal endmay include a Clark sensor to generate a signal as a function of thepartial pressure of oxygen within the cerebrospinal fluid. Embodimentsin which an IMD comprises an implantable pump, for example, may includea catheter with a distal portion located in the cerebrospinal fluid.

In some embodiments, sensors 40 may include one or more intraluminal,extraluminal, or external flow sensors positioned to generate a signalas a function of arterial blood flow. A flow sensor may be, for example,an electromagnetic, thermal convection, ultrasonic-Doppler, orlaser-Doppler flow sensor. Further, in some external medical deviceembodiments of the invention, sensors 40 may include one or moreelectrodes positioned on the skin of patient 12 to generate a signal asa function of galvanic skin response.

Processor 46 monitors the physiological parameters based on the signalsgenerated by the one or more sensors 40, and determines whether patient12 is asleep based on current values for the physiological parameters.In some embodiments, processor 46 may determine mean or median value forthe parameter based on values of the signal over time, and determineswhether patient 12 is asleep based on the mean or median value. In otherembodiments, processor 46 may additionally or alternatively determine avariability of one or more of the parameters based on the values of theparameter over time, and may determine whether patient 12 is asleepbased on the one or more variability values. IMD 14 may includecircuitry (not shown) that conditions the signals generate by sensors 40such that they may be analyzed by processor 46. For example, IMD 14 mayinclude one or more analog to digital converters to convert analogsignals generate by sensors 40 into digital signals usable by processor46, as well as suitable filter and amplifier circuitry.

In some embodiments, processor 46 determines a value of a sleep metricthat indicates a probability of the patient being asleep based on aphysiological parameter. In particular, processor 46 may apply afunction or look-up table to the current value, mean or median value,and/or variability of the physiological parameter to determine the sleepmetric value. For example, the values of one or more physiologicalparameters serve as indices to the lookup table to yield a correspondingoutput value, which serves as the sleep metric value. Processor 46 maycompare the sleep metric value to a threshold value to determine whetherpatient 12 is asleep. In some embodiments, processor 46 may compare thesleep metric value to each of a plurality of thresholds to determine thecurrent sleep state of patient 12, e.g., rapid eye movement (REM), S1,S2, S3, or S4.

Further, in some embodiments processor 46 determines a sleep metricvalue for each of a plurality of monitored physiological parameters. Inother words, processor 46 may apply a function or look-up table for eachparameter to the current value for that parameter in order to determinethe sleep metric value for that individual parameter. Processor 46 maythen average or otherwise combine the plurality of sleep metric valuesto provide an overall sleep metric value, and may determine whetherpatient 12 is asleep based on the overall sleep metric value. In someembodiments, processor 46 may apply a weighting factor to one or more ofthe sleep metric values prior to combination.

As shown in FIG. 3, memory 48 may include parameter information 60recorded by processor 46, e.g., parameter values, or mean or medianparameter values. Memory 48 may also store sleep metric functions 62 orlook-up tables (not shown) that processor 46 may retrieve forapplication to physiological parameter values or variability values, andthreshold values 64 that processor 46 may use to determine whetherpatient 12 is asleep and, in some embodiments, the sleep state ofpatient 12. Memory 48 may also store weighting factors 66 used byprocessor 46 when combining sleep metric values to determine an overallsleep metric value. Processor 46 may store sleep information 68 withinmemory 48, such as recorded sleep metric values and informationindicating when patient 12 was asleep.

As shown in FIG. 2, IMD 14 also includes a telemetry circuit 50 thatallows processor 46 to communicate with clinician programmer 20 andpatient programmer 26. For example, using clinician programmer 20, aclinician may direct processor 46 to retrieve sleep information 68 frommemory 48 and transmit the information via telemetry circuit 50 toprogrammer 20 for analysis. Further, the clinician may select or adjustthe one or more of functions 62, look-up tables, thresholds 64 andweighting factors 66 in order to select or adjust the sensitivity andspecificity of processor 46 determining whether the patient is asleep.

FIG. 4 is a flowchart illustrating an example technique for determiningwhether a patient is asleep that may be employed by IMD 14. According tothe example technique, IMD 14 monitors a plurality of physiologicalparameters of patient 12 (70). More particularly, processor 46 receivessignals from one or more sensors 40, and monitors the physiologicalparameters based on the signals.

Processor 46 applies a respective function 62 to current values, mean ormedian values, and/or variability values for each of physiologicalparameters to determine a sleep metric value for each of the parameters(72). Processor 46 then combines the various sleep metric values todetermine a current overall sleep metric value (74). For exampleprocessor 46 may apply weighting factors 66 to one or more of theparameter specific sleep metric values, and then average the parameterspecific sleep metric values in light of the weighting factors 66.

Processor 46 compares the current overall sleep metric value to athreshold value 64 (76), and determines whether patient 12 is asleep orawake, e.g., whether the sleep state of patient 12 has changed, based onthe comparison (78). For example, processor 46 may determine thatpatient 12 is asleep if the current overall sleep metric value exceedsthe threshold value 64. If the sleep state of patient 12 has changed,processor 46 may initiate, suspend or adjust a therapy delivered topatient 12 by IMD 14, or processor 46 may store an indication of thetime and the change within memory 48 (80).

Various embodiments of the invention have been described. However, oneskilled in the art will appreciated that various modifications may bemade to the described embodiments without departing from the scope ofthe invention. For example, although described herein in the context ofan implantable neurostimulator, the invention may be embodied in anyimplantable or external medical device.

As another example, although described in the context of determiningwhether a patient is asleep, e.g., whether the patient's current sleepstate is asleep or awake, the invention may, as described above, be usedto determine what level of sleep a patient is currently experiencing,e.g., which of sleep states REM, S1, S2, S3, and S4 the patient iscurrently in. A medical device may record transitions between thesestates and between sleep and wakefulness, or may control therapy basedon transitions between these states and between sleep and wakefulness.Further, in some embodiments, a medical device may, without making asleep determination, simply record one or more determined sleep metricvalues for later analysis, or may control delivery of therapy based onthe sleep metric values.

Further, the invention may be embodied in a programming device, such asprogrammers 20, 26 described above. In particular, in some embodiments,a programming device may determine when patient 12 is asleep accordingto the invention instead of, or in addition to an implantable orexternal medical device. For example, a medical device may record valuesfor one or more of the physiological parameters discussed herein, andmay provide the physiological parameter values to the programming devicein real time or when interrogated by the programming device. Theprogramming device may apply the techniques described herein withreference to IMD 14 to determine when patient 12 is asleep, and maycontrol delivery of therapy based on the determination, or presentinformation relating to the patient's sleep patterns to a user to enablediagnosis or therapy evaluation.

The invention may also be embodied as a computer-readable medium, suchas memory 48, that includes instructions to cause a processor, such asprocessor 46, to perform any of the methods described herein. These andother embodiments are within the scope of the following claims.

1. A method comprising: monitoring a plurality of physiologicalparameters of a patient; and determining a value of a sleep metric thatindicates a probability of the patient being asleep based on thephysiological parameters.
 2. The method of claim 1, wherein thephysiological parameters comprise at least one of activity level,posture, heart rate, respiration rate, respiratory volume, and coretemperature.
 3. The method of claim 1, wherein the physiologicalparameters comprise at least one of blood pressure, blood oxygensaturation, partial pressure of oxygen within blood, partial pressure ofoxygen within cerebrospinal fluid, muscular activity, core temperature,arterial blood flow, and galvanic skin response.
 4. The method of claim1, further comprising determining a variability of at least one of thephysiological parameters, wherein determining a value of the sleepmetric comprises determining the value of the sleep metric based on thevariability.
 5. The method of claim 1, further comprising determining atleast one of a mean value and a median value for at least one of thephysiological parameters, wherein determining a value of a sleep metriccomprises determining the sleep metric based on the at least one of themean value and the median value.
 6. The method of claim 1, whereindetermining a value of a sleep metric comprises determining a value foreach of a plurality of sleep metrics, each of the values determinedbased on a respective one of the physiological parameters.
 7. The methodof claim 6, wherein determining a value of a sleep metric comprisesdetermining a value of an overall sleep metric based on the plurality ofsleep metric values.
 8. The method of claim 7, wherein determining avalue of an overall sleep metric comprises averaging the values of theplurality of sleep metrics.
 9. The method of claim 7, whereindetermining a value of an overall sleep metric comprises applying aweighting factor to a value of at least one of the plurality of sleepmetrics.
 10. The method of claim 1, further comprising: comparing thevalue of the sleep metric to a threshold value; and determining whetherthe patient is asleep based on the comparison.
 11. The method of claim10, further comprising: comparing the value of the sleep metric to aplurality of thresholds; and determining a sleep state of the patientbased on the comparison.
 12. The method of claim 11, wherein determininga sleep state of the patient comprises determining whether the patientis in one of a rapid eye movement (REM) sleep state and a nonrapid eyemovement (NREM) sleep state.
 13. The method of claim 10, wherein thethreshold value is selected by a user.
 14. The method of claim 10,further comprising controlling delivery of a therapy to the patientbased on the determination of whether the patient is asleep.
 15. Themethod of claim 10, further comprising storing information indicatingwhen the patient is asleep for retrieval by a user.
 16. The method ofclaim 15, further comprising evaluating the effectiveness of a therapydelivered to the patient based on the information indicating whether thepatient is asleep.
 17. The method of claim 16, wherein the therapycomprises at least one of a neurostimulation and a drug therapy.
 18. Themethod of claim 16, wherein the therapy is a pain therapy.
 19. A medicalsystem comprising: a plurality of sensors, each of the sensorsgenerating a signal as a function of at least one physiologicalparameter of a patient; and a processor that monitors a plurality ofphysiological parameters of the patient based on the signals output bythe sensors, and determines a value of a sleep metric that indicates aprobability of the patient being asleep based on the physiologicalparameters.
 20. The system of claim 19, wherein the physiologicalparameters comprise at least one of activity level, posture, heart rate,respiration rate, respiratory volume, and core temperature.
 21. Thesystem of claim 19, wherein physiological parameters comprise at leastone of blood pressure, blood oxygen saturation, partial pressure ofoxygen within blood, partial pressure of oxygen within cerebrospinalfluid, muscular activity, arterial blood flow, and galvanic skinresponse.
 22. The system of claim 19, wherein the processor determines avariability of at least one of the physiological parameters, anddetermines the sleep metric based on the variability.
 23. The system ofclaim 19, wherein the processor determines at least one of a mean valueand a median value of at least one of the physiological parameters, anddetermines the sleep metric based on the at least one of the mean valueand the median value.
 24. The system of claim 19, wherein the processordetermines a value of each of a plurality of sleep metrics, each of theplurality of values determined based on a respective one of thephysiological parameters.
 25. The system of claim 24, wherein theprocessor determines a value of an overall sleep metric based the valuesof the plurality of sleep metrics.
 26. The system of claim 25, whereinthe processor determines the value of the overall sleep metric byaveraging the values of the plurality of sleep metrics.
 27. The systemof claim 26, wherein the processor applies a weighting factor to atleast one of values of the plurality of sleep metrics.
 28. The system ofclaim 19, further comprising a memory to store a threshold value,wherein the processor compares the value of the sleep metric to athreshold value and determines whether the patient is asleep based onthe comparison.
 29. The system of claim 28, wherein the memory stores aplurality of threshold values, and the processor compares the value ofthe sleep metric to each of the threshold values and determines a sleepstate of the patient based on the comparison.
 30. The system of claim29, wherein the processor determines whether the patient is in one of arapid eye movement (REM) sleep state and a nonrapid eye movement (NREM)sleep state.
 31. The system of claim 28, further comprising a userinterface, wherein a user selects the threshold via the user interface.32. The system of claim 28, wherein the processor controls delivery of atherapy to the patient based on the determination of whether the patientis asleep.
 33. The system of claim 28, wherein the processor storesinformation indicating when the patient is asleep within the memory forretrieval by a user.
 34. The system of claim 19, further comprising animplantable medical device that includes the processor.
 35. The systemof claim 34, wherein the implantable medical device includes the sensor.36. The system of claim 34, wherein the implantable medical device iscoupled to the sensor via a lead.
 37. The system of claim 34, whereinthe implantable medical device is wirelessly coupled to the sensor. 38.The system of claim 34, wherein the implantable medical device comprisesat least one of an implantable neurostimulator and an implantable pump.39. A system comprising: means for monitoring a plurality ofphysiological parameters of a patient; and means for determining a valueof a sleep metric that indicates a probability of the patient beingasleep based on the physiological parameters.
 40. The system of claim39, further comprising means for generating at least one signal as afunction of the physiological parameters, wherein the means formonitoring comprises means for monitoring the physiological parametersbased on the signal.
 41. The system of claim 39, wherein the means fordetermining a sleep metric comprises means for determining a value foreach of a plurality of sleep metrics, each of the plurality of valuesdetermined based on a respective one of the physiological parameters.42. The system of claim 41, wherein the means for determining a value ofa sleep metric comprises means for determining a value of an overallsleep metric based the values of the plurality of sleep metrics.
 43. Thesystem of claim 41, further comprising means for comparing the value ofthe sleep metric to a threshold value and determining whether thepatient is asleep based on the comparison.
 44. The system of claim 43,further comprising: means for delivering a therapy to the patient; andmeans for controlling delivery of a therapy to the patient by thetherapy delivery means based on the determination of whether the patientis asleep.
 45. The system of claim 43, further comprising means forstoring information indicating when the patient is asleep for retrievalby a user.
 46. A method comprising: monitoring a physiological parameterof a patient via an implantable medical device, wherein thephysiological parameter comprises one of blood pressure, blood oxygensaturation, muscular activity, core temperature, arterial blood flow,and galvanic skin response; and determining whether a patient is asleepbased on the physiological parameter.
 47. The method of claim 46,wherein monitoring a physiological parameter comprises monitoring aplurality of physiological parameters, and wherein determining whether apatient is asleep comprises determining whether the patient is asleepbased on the plurality of physiological parameters.
 48. The method ofclaim 47, wherein determining whether a patient is asleep comprises:determining a value of a sleep metric that indicates a probability ofthe patient being asleep based on the plurality of physiologicalparameters; comparing the value of the sleep metric to a thresholdvalue; and determining whether the patient is asleep based on thecomparison.
 49. The method of claim 48, wherein determining a sleepmetric comprises: determining a value of a sleep metric for each of theplurality of monitored physiological parameters; and determining a valueof an overall sleep metric based on the values of the plurality of sleepmetrics, and wherein determining whether the patient is asleep comprisesdetermining whether the patient is asleep based on comparison of thevalue of the overall sleep metric to the threshold.
 50. The method ofclaim 46, further comprising controlling delivery of a therapy to thepatient based on the determination of whether the patient is asleep. 51.The method of claim 46, further comprising storing informationindicating when the patient is asleep for retrieval by a user.
 52. Themethod of claim 51, further comprising evaluating the effectiveness of atherapy delivered to the patient based on the information indicatingwhether the patient is asleep.
 53. A medical system comprising: a sensorto generate a signal as a function of a physiological parameter of apatient, wherein the physiological parameter comprises one of bloodpressure, blood oxygen saturation, muscular activity, core temperature,arterial blood flow, and galvanic skin response; and an implantablemedical device that includes a processor to monitor the physiologicalparameter based on the signal and determine whether a patient is asleepbased on the physiological parameter.
 54. The system of claim 53,further comprising a plurality of sensors that generate a signal as afunction of a physiological parameter of a patient, wherein theprocessor monitors a plurality of physiological parameters based on thesignals and determines whether the patient is asleep based on theplurality of physiological parameters.
 55. The system of claim 54,wherein the implantable medical device further comprises a memory tostore a threshold value, and wherein the processor determines a value ofa sleep metric that indicates a sleep state of the patient based on theplurality of physiological parameters, compares the value of the sleepmetric to a threshold value, and determines whether the patient isasleep based on the comparison.
 56. The system of claim 55, wherein theprocessor determines a value of a sleep metric for each of the pluralityof monitored physiological parameters, determines a value of an overallsleep metric based on the values of the plurality of sleep metrics, anddetermines whether the patient is asleep based on comparison of thevalue of the overall sleep metric to the threshold.
 57. The system ofclaim 53, wherein the processor controls delivery of a therapy to thepatient based on the determination of whether the patient is asleep. 58.The system of claim 53, wherein the implantable medical device furthercomprises a memory, and wherein the processor stores informationindicating when the patient is asleep within the memory for retrieval bya user.
 59. The system of claim 53, wherein the implantable medicaldevice includes the sensor.
 60. The system of claim 53, wherein theimplantable medical device is coupled to the sensor via a lead
 61. Thesystem of claim 53, wherein the implantable medical device is wirelesslycoupled to the sensor.
 62. The system of claim 53, wherein theimplantable medical device comprises at least one of an implantableneurostimulator and an implantable pump.
 63. A computer-readable mediumcomprising instructions that cause a programmable processor to: monitora plurality of physiological parameters of a patient; and determine avalue of a sleep metric that indicates a probability of the patientbeing asleep based on the physiological parameters.
 64. The medium ofclaim 63, further comprising instructions that cause the processor todetermine a variability of at least one of the physiological parameters,wherein the instructions that cause the processor to determine a valueof the sleep metric comprise instructions that cause the processor todetermine the value of the sleep metric based on the variability. 65.The medium of claim 63, further comprising instructions that cause theprocessor to: determine a value for each of a plurality of sleepmetrics, each of the values determined based on a respective one of thephysiological parameters; and determine a value of an overall sleepmetric based on the plurality of sleep metric values.
 66. The medium ofclaim 63, further comprising instructions that cause the processor to:compare the value of the sleep metric to a threshold value; anddetermine whether the patient is asleep based on the comparison.
 67. Themethod of claim 66, further comprising instructions that cause theprocessor to: compare the value of the sleep metric to a plurality ofthresholds; and determine a sleep state of the patient based on thecomparison.
 68. The medium of claim 67, wherein the instructions thatcause the processor to determine a sleep state of the patient compriseinstructions that cause the processor to determine whether the patientis in one of a rapid eye movement (REM) sleep state and a nonrapid eyemovement (NREM) sleep state.